In order to accomplish a safe control of a moveable object such as an industrial robot it is essential to have absolute confidence in the control of the object. As an object in this context should be understood any stationary or mobile moveable object. The operator must be fully confident that the commanded order is effected. Any other person or machine must not be able to interfere with the control of the object on purpose or by accident.
In a system where an operating unit is connected to a control unit via a cable this is easy. Such a cable is preferably shielded and no other communication system can interfere with the two connected units. The cable is however heavy and stiff thus making the operation of the operating unit cumbersome. Another problem with a cable connection is that the cable often is getting entangled in loops thus making it difficult to stretch it out. When a plurality of robots are placed together in a cell the cables often are entangled themselves thus making it difficult to find out which operating unit belongs to which robot. Finally there is the risk of the cable being damaged especially when a vehicle is running over it. If the cable is damaged it is involving much work to have it repaired or exchanged. It is therefore a long lasting desire in the industry of industrial robots to carry out robot control without a cable.
The obvious solution is a wireless control system. The need described above then will be very obviously solved. Controlling an object with a wireless system where the air could contain a plurality of simultaneous command messages involves, however, other problems. The receiving part of a specific control system must find out and recognize a control message addressed to that specific control unit and effectuate that command. It is in such an environment obvious that sometimes different signals will interfere with each other. It is also obvious as the operating unit is moved around that sometimes the aerial will be located in a “radio shadow” thus no signal from the unit will be received by the control unit. In such cases there must be a control system which accounts for time slots where no signal is received and for a correct action accordingly.
From “Gebrauchsmuster” DE 297 10 026 U1 a wireless control of an industrial robot is previously known. A mobile operating unit has a wheel knob that sends pulses via a Digital European Cordless Telephone (DECT) module to a numeric control device. The safety aspects of this wireless control is not mentioned in the document. There is thus nothing mentioned on how to proceed when a signal is not received by the receiving part.
Another wireless control system is previously known from JP 11073201. The problem solved in this document is the communication of an emergency stop control carried out by wireless means in a manner equivalent to a cable communication. Under normal conditions the controller periodically transmits data to the operating unit. Upon receiving this data the operating unit responds by transmitting a response signal back to the controller. Thus the control unit asks the operating unit if this is in function. The reason for making the communication intermittent is that data, if transmitted and received at every time, increases the electric power consumption of the battery-operated teaching unit. The length of a time period however is not mentioned. The controller judges from said response signal that the operating unit is in normal condition. If the emergency switch has been activated the response signal carries that information and the controller executes the emergency stop.
Under abnormal situations, that means when the operating unit is not in a correct function mood, the operating unit transmits no response signal. Based on the absence of a response from the operating unit the controller outputs the conclusion that there is an “emergency stop condition”.
From JP 9117888 another control system is known the object of which is to provide a remote control with an emergency stop function. The system is said to ensure safety while cutting costs by making possible to carry out the remote control of a robot etc by transmitters for wireless remote controllers or by serial wired communication means. According to the system an operator is actuating a control permit switch by means of a key operation. A base signal is then continuously transmitted from the transmitter of the operating unit. When driving control switches are activated a driving control signal is transmitted in addition to the base signal.
A control part of the system is provided in the object to be controlled. When a continuous base signal is received and driving control signals are received in addition to this the driving means is then controlled. When, however, the base signal is interrupted the driving is stopped irrespective of the existence of other signals. Thus an emergency stop function is achieved by reliably stopping the driving means when the actuation of said control permit switch is released.
The signal is sent in time slots, which has a typical length of 108 ms, and contain a leader code (A), a control permit code (C) also known as base signal and a switch code (B). In the absence of the base signal the power source of the driving control means is cut of and the motor is stopped irrespective of the other signals. Thus when the need for an emergency stop arises the motor is reliably stopped when the operator stops operating the key of the servo control permit switch. Also when the signal between the transmitter and the robot is cut off or when the transmission is poor the power source is cut of from the driving control means thus effecting an emergency stop function.
Both of these Japanese documents describe the execution of an emergency stop in the absence of a base signal sent from an operating device. However, an emergency stop is not wanted when a signal from the operating unit is just lacking for a short moment. There will always be moments when such a base signal is not received for one or more time slots. This, however, does not call for an emergency stop execution.
When an emergency stop is executed the whole robot cell or even several cells will be cut of from electric power. To reset an emergency stop a plurality of checks has to be made and the cell will be out of work for a considerable time. This is therefore a not wanted situation.